It will also be found that the horn-like projection of the pupa consists of a right and left division, one belonging to each maxilla: that its labium consists of two back-wardly turned lobes united basally, and of great size in Pieris: that the coxa of the limbs are united to the thorax, the trochanter inconspicuous: that the femur and tibia are bent at an angle on one another, the former concealed by the latter, and that the joints of the tarsus are not differentiated. Though the first abdominal spiracle is hidden by the wings, it retains the character of an open functional spiracle.
If the horn-like projection of Sphinx Ligustri is opened when the moth is nearly ready to emerge, the bases of the antlia of the imago will be found to fill it imperfectly. Each base forms a thick band or ribbon attached anteriorly to the head, lying under the outer surface of the projection, inside which it is folded back once upon itself. It then runs on into the straight median portion of the pupal maxilla. It appears to me likely that it is differentiated from a part, and not the whole of the outer wall of the pupal maxilla, but the histological details of the process are still wanting. Owing to the fact that the antennae, antliae, etc, of the imago are formed within the corresponding organs of the pupa, and are withdrawn from them leaving them empty when the imago emerges, the pupal organs have been spoken of as 'cases' or 'thecae,' e.g. Ceratotheca, Glossotheca, etc. But it must be borne in mind that just as the change from caterpillar to pupa takes place by a moult, and the pupal organs are formed within the corresponding organs of the caterpillar, from which they differ essentially in shape and size, so it is with the change from pupa to imago. Indeed there is reason to believe that more than one moult takes place during the pupal stage.
In Sphinx Ligustri and in some others (?) a thin pellicle may be raised from the inner surface of the last pupal skin; and Professor Westwood has drawn my attention to a passage in Curtis (British Entomology, Description of Plate 147), where that author records the fact that an imago of Acherontia Atropos cast off a complete and thin pellicle after emergence from the pupa-skin. The pellicle in question appears to be homologous with the thin skin cast by the sub-imago of Ephemeridae after it has taken flight from the water, having already just emerged from another skin.
The pupal state of Sphinx Ligustri lasts for forty-two to forty-three weeks. During this period changes take place affecting all the internal organs. Changes in the nervous system continue for the first four weeks, but are then suspended until March. They have been worked out in this moth and in Vanessa Urticae by Newport; whilst Herold has worked them out in Pieris Brassicae, as well also as the changes undergone by the digestive system and the evolution of the male and female organs.
In some pupae, e.g. of Cossus, the edges of the abdominal somites are fringed with short spines or adminicula to aid the movements of the animal. The apex of the cremaster varies much in character. The duration of the pupal state differs much. In small species it lasts only a few days. In Lepidoptera with two broods in a year, e. g. Papilio Machaon, the pupal state of the first brood lasts thirteen days, of the second from September to June. In the broods of various Pieridae, etc, the same differences may be observed. The pupal state of Sphinx Ligustri is occasionally extended for a year beyond the normal: and this is often the case in some other genera. It is a very common thing for a larval Tenthredinidan (Hymenoptera) to delay assuming the pupal state within its cocoon, but it is an extremely rare occurrence in Lepidoptera. It is said, however, that if the Cossus larva makes its cocoon in autumn, the caterpillar does not become a pupa till after winter has passed: if it makes it in June, it becomes a pupa at once and emerges as an imago in three to four weeks.
The wings make their first (i. e. outward) appearance in the pupa stage of insects with a perfect metamorphosis. In the newly-formed Lepidopterous pupa they are hollow sacs with a cavity continuous with the coelome. These sacs swell out as the larval skin is being cast. But wings appear from the first as external processes, gradually increasing in size with successive moults in insects such as the Earwig, Ephemeron, Cockroach, i.e. in Insecta Ametabola, and Hemimetabola. Dewitz (Berlin. Entomol. Zeitung, xxv. 1881) has found in very early stages of larval Trichoptera and Lepidoptera a pair of small meso- and meta-thoracic involutions of the hypodermis cells containing an internal chitinous lamella continuous with the cuticle. These involutions increase in size at every successive moult, and acquire a more or less perfect investment of mesodermic cells derived from the sheaths of either the tracheae or the nerves. They are evaginated previously to the last moult by the withdrawal of the internal chitinous lamella, and when the last larval skin is stripped off they appear as external sacs (supra).
The homology and first origin of wings are points of great obscurity. There are no traces of them in the most primitive Insectan order known, the Thysanura, a survival of forms existing apparently before the acquisition of wings. Wings are to be considered as secondary or acquired structures. It is tempting to regard them as modified tracheal gills, which they much resemble in structure, and which are also organs secondarily acquired. But it is impossible to suppose that all Insecta with a very limited number of exceptions are descended from ancestors which took to aquatic habits; 'took because the first Tracheata were without doubt terrestrial forms. And the only supposition that appears feasible is that respiratory structures similar to tracheal gills were of use to terrestrial Insecta living under conditions long passed away. The larval Calotermes rugosus (F. Miiller, J. Z. ix. 1875), one of the Termitidae, animals of subterranean habit, develops peculiar dorsal appendages devoid of tracheae on the pro- and meso-thorax. The pair on the prothorax disappears: that on the mesothorax acquires tracheae, and grows into the mesothoracic pair of wings. The metathoracic pair of wings develops in a similar manner but at a later period.
It must be remembered that the oldest fossil Insecta known, even the Silurian Blatta, possess wings, so far as can be judged, of the ordinary structure. The first origin of these organs dates back therefore to a period and to conditions of which we have no record.
Pupa and Imago of Cossus.Lyonet, Recherches sur l'anatomie et les metamorphoses de differentes especes d'Insectes. Ouvrage posthume, Paris, 1832.
Changes in nervous and digestive systems, and Evolution of reproductive organs. Herold, Entwickelungsgeschichte der Schmetterlinge, Cassel, 1815. Changes in nervous system. Newport, Ph. Tr. 1832 and 1834. In reproductive. Bessels, Z. W. Z. xvii. 1867.
Formation of antennae of Imago.Dewitz, Biol. Centralbl. iii. 1882-3. Of wing, Id. Berliner. Entomol. Zeitung, xxv. 1881. Pancritius, Z. A. vii. 1884.
Philosophy of Pupation.Riley, American Entomologist, iii. 1880.